Lewis & Short

extĕnŭātĭo, ōnis, f. [extenuo], a thinning, rarefaction.

  1. I. Lit.: vitium, Plin. 17, 26, 39, § 246; Sen. Q. N. 2, 57, 3.
  2. II. Trop., a lessening, diminution, extenuation, as a figure of speech, the Gr. μείωσις or ἐλάττωσις, Cic. de Or. 3, 53, 202; Quint. 9, 1, 28; 9, 2, 3.